This invention relates to a thin film-type magnetic disk which is used, for example, in a magnetic memory for a computer and is suited for high-density recording.
As a thin film-type magnetic disk using a thin film of metal or metal oxide as a magnetic layer, there has been practically used one which usually comprises a non-magnetic substrate formed by subjecting an anodizing treatment, Ni.multidot.P plating or the like to a surface of an aluminum alloy plate, and a thin film magnetic layer formed on the substrate by a vacuum thin film-forming or deposition process or a plating process. Generally, there is employed a so-called contact start-stop (CSS) system in which a magnetic head floats or lifts away from the disk surface at the time of start of the magnetic disk, and contacts the disk surface at the time of stop of the magnetic disk. In this case, in order that the disk surface can withstand a sliding contact with the magnetic head and a slider at the time of start or stop of the magnetic disk, and also that the magnetic head and the slider can be prevented from adhering to the disk surface due to adsorbed water or condensed water in a highly humid environment when the magnetic disk is stopped (this phenomenon is a so-called adhesion phenomenon), and further that a magnetic anisotropy can be enhanced to secure a high output, the disc surface is usually formed into a corrugated or undulatory configuration having bulged portions extending in the direction of movement of the magnetic head relative to the disk. The corrugated configuration is formed as a result of a texture processing applied to the substrate surface. Further, a protective film made mainly of graphite, SiO.sub.2 or the like is formed on the magnetic thin film, and a lubricant layer is formed on the protective film.
In order to improve a recording density, it is necessary to reduce the floating distance of the magnetic head. In this case, if the surface properties are improved by reducing the height of the bulged portions provided by the texture processing, the CSS withstanding ability and the ability of preventing the adhesion of the magnetic head are correspondingly sacrificed. As a result, it is difficult to secure a satisfactory practical performance.
The texture processing is generally to produce fine scars in the surface of the disk substrate by abrasion using abrasive grains, the scars being distributed in the circumferential direction of the disk substrate surface. Its bulged configuration has a very gentle gradient in the circumferential direction of the magnetic disk much like a ridge or crest of a mountain. The configuration of such a gentle gradient, when it receives an impact force from the magnetic head, is liable to be deformed and broken, and is liable to be in face contact when the magnetic disk slides relative to the magnetic head, thus exhibiting a higher coefficient of friction as compared with a point contact.
In view of the foregoing, an extensive study has been made, and it has now been concluded that the best means for reducing the floating distance of the magnetic head and for securing a practical reliability is to reduce the height of the circumferentially-extending bulged portions and also to provide a number of fine particle-like projections of a sharp gradient at least on that portion of each bulged portion which is in the vicinity of the top of the bulged portion and contacts the magnetic head, thereby providing a point contact between the magnetic head and the surface of the magnetic disk.
Also, it has now been found that similar effects can be achieved by removing the circumferentially-extending bulged portions and by providing groups of fine particle-like projections of a sharp gradient in narrow bands in the circumferential direction of the disk. However, it has been found that when such fine particle-like projections are distributed over the entire flat smooth surface having no bulged portion, there arises another problem that the head adhesion phenomenon occurs.